The present invention relates to a photomask for forming a fine pattern used for producing a semiconductor integrated circuit device, a method for producing the same and a method for forming a pattern using the photomask.
In recent years, it is increasingly necessary to miniaturize circuit patterns for high integration of a large-scale integrated circuit device (hereinafter, referred to as “LSI”) that can be realized with semiconductors. As a result, a reduction of the width of a line for wiring patterns constituting a circuit or miniaturization of contact hole patterns (hereinafter, referred to as “contact patterns”) that connect between layered wirings formed via insulating layers have become very important.
Hereinafter, miniaturization of wiring patterns with a recent light-exposure system will be described by taking the case of using a positive resist process as an example. In a positive resist process, a line pattern refers to a line-shaped resist film (resist pattern) that are left, corresponding to a non-exposed region of a resist by exposure with a photomask and subsequent development. A space pattern refers to a portion from which a resist is removed (resist-removed pattern) corresponding to an exposed region of a resist. A contact pattern refers to a hole-like resist-removed portion and can be regarded as a small space pattern of the space patterns. When using a negative resist process instead of a positive resist process, the definition of the line pattern and the definition of the space pattern are replaced by each other.
In general, for miniaturization of wiring patterns, a method for forming a fine line pattern with oblique incident light exposure (off-axis illumination) called super resolution exposure has been used. This method is an excellent method for miniaturization of a resist pattern corresponding to a non-exposed region of a resist, and also has an effect of improving the depth of focus of dense patterns that are arranged periodically. However, this oblique incident exposure method has little effect on miniaturization of isolated resist-removed portions, and on the contrary, this method deteriorates the contrast of images (optical images) and the depth of focus. Therefore, the oblique incident exposure method is positively used to form patterns characterized in that the size of the resist-removed portion is larger than the size of a resist pattern, for example, to form gate patterns.
On the other hand, to form a micro resist-removed portion that is isolated such as a small contact pattern, it is known that it is useful to use a small light source having a low coherence degree that contains no oblique incident component. In this case, it is more useful to use a half-tone phase-shifting mask (see, for example, Japanese Laid-Open Patent Publication No. 9-90601). In the half-tone phase-shifting mask, a phase sifter that has a very low transmittance of about 3 to 6% with respect to exposure light and that causes phase inversion of 180 degrees with respect to light transmitted through an opening, instead of a complete light-shielding portion, is provided as a mask pattern surrounding a light-transmitting portion (opening) corresponding to a contact pattern.
In this specification, a transmittance is represented by an effective transmittance when the transmittance of a transparent substrate is taken as 100%, unless otherwise specified. Moreover, “complete light-shielding film (complete light-shielding portion) refers to a light-shielding film (light-shielding portion) having an effective transmittance of smaller than 1%.
Hereinafter, the principle of the method for forming patterns using a half-tone phase-shifting mask will be described with reference to FIGS. 27A to 27G.
FIG. 27A is a plan view of a photomask in which an opening corresponding to a contact pattern is provided in a chromium film serving as a complete light-shielding portion provided on the surface of the mask. FIG. 27B shows the amplitude intensity corresponding to line AA′ of light transmitted through the photomask shown in FIG. 27A. FIG. 27C is a plan view of a photomask in which a chromium film corresponding to a contact pattern as a complete light-shielding portion is provided in a phase shifter provided on the surface of the mask. FIG. 27D shows the amplitude intensity corresponding to line AA′ of light transmitted through the photomask shown in FIG. 27C. FIG. 27E is a plan view of a photomask in which an opening corresponding to a contact pattern is provided in a phase shifter provided on the surface of the mask (i.e., a half-tone phase-shifting mask). FIGS. 27F and 28G show the amplitude intensity and the light intensity corresponding to line AA′ of light transmitted through the photomask shown in FIG. 27E, respectively.
As shown in FIGS. 27B, 27D, and 27F, the amplitude intensity of light transmitted through the half-tone phase-shifting mask shown in FIG. 27E is equal to the sum of the amplitude intensities of lights transmitted through the photomasks shown in FIGS. 27A and 27C. That is to say, in the half-tone phase-shifting mask shown in FIG. 27E, the phase shifter serving as a light-shielding portion is configured so as to not only transmit light at a low transmittance, but also provide an optical path difference (phase difference) of 180 degrees with respect to the light transmitted through the opening to the light transmitted through this phase shifter. Therefore, as shown in FIGS. 27B and 27D, the light transmitted through the phase shifter has an amplitude intensity with a phase opposite to that of the light transmitted through the opening. Thus, if the amplitude intensity distribution shown in FIG. 27B and the amplitude intensity distribution shown in FIG. 27D are synthesized, a phase boundary in which the amplitude intensity is turned to 0 by a phase change is generated, as shown in FIG. 27F. As a result, as shown in FIG. 27G, in the end of the opening that is the phase boundary (hereinafter, referred to as a “phase end”), the light intensity, which is represented by a square of the amplitude intensity, becomes 0, and a significantly dark portion is formed. Accordingly, in an image of the light transmitted through the half-tone phase-shifting mask shown in FIG. 27E, strong contrast is realized in the vicinity of the opening. However, the following should be noted: This improvement of the contrast occurs with respect to light vertically incident to the mask, more specifically, that is, light incident to the mask from a small light source region having a low coherence degree. However, the contrast is not improved even in the vicinity of the opening (in the vicinity of the phase boundary in which a phase change occurs) with respect to oblique incident exposure light, for example, exposure called annular illumination in which a vertical incident component (illumination component from the center of a light source (the normal direction of the mask) is removed. Furthermore, there is another disadvantage in that compared with the case where exposure is performed with a small light source having a low coherence degree, the depth of focus is lower in the case where oblique incident exposure is performed.
As described above, in order to form a fine resist-removed pattern such as a contact pattern using a positive resist process, it was necessary to perform exposure with a small light source having a coherence degree of about 0.5 or less, which provides illumination only with vertical incident components, in combination with a half-tone phase-shifting mask. This method was very useful to form fine and isolated contact patterns.
There is a recent tendency associated with a high degree of integration of recent semiconductor devices that densely arranged patterns as well as isolated patterns are also required not only for wiring patterns but also contact patterns. In order to realize a high depth of focus when forming densely arranged contact patterns, oblique incident exposure is useful as in the case of the densely arranged wiring patterns.
Furthermore, in recent years, also when forming wiring patterns, in addition to miniaturization of line patterns serving as wiring patterns, there is an increasing demand for miniaturization of space patterns between wirings. As in the case of the isolated contact patterns, it is useful to use a light source having a low coherence degree in combination with a half-tone phase-shifting mask in order to form small isolated space patterns between wirings.
That is to say, although oblique incident exposure is essential to form high density wiring patterns and high density contact patterns, the contrast and the depth of focus of isolated contact patterns and isolated space patterns between wirings are significantly deteriorated when oblique incident exposure is performed. The contrast and the depth of focus are deteriorated even more significantly when a half-tone phase-shifting mask is used to improve the resolution.
On the other hand, when a small light source having a low coherence degree is used to form small isolated contact patterns and small isolated space patterns between wirings, it becomes difficult to form high density patterns or small line patterns.
Therefore, the optimal illumination conditions with respect to small isolated space patterns and the optical illumination conditions with respect to densely arranged patterns or small line patterns have a contradictory relationship. Therefore, in order to form small resist patterns and small isolated resist-removed patterns at the same time, a light source having a medium coherence degree (about 0.5 to 0.6) is used for a trade-off between the effect of vertical incident components from a light source and the effect of oblique incident components from a light source. However, in this case, both the effect of vertical incident components and the effect of oblique incident components are canceled, so that it is difficult to realize further high integration of semiconductor devices by miniaturizing isolated line patterns or densely arranged patterns and isolated space patterns at the same time.